1. Field of the Invention
This invention relates to purification of siloxanes, and more particularly, to a method of readily removing ionic crystal impurities from siloxanes.
2. Prior Art
One well-known prior art method for producing high molecular weight organopolysiloxanes is by converting relatively low molecular weight organopolysiloxanes into relatively high molecular weight organopolysiloxanes through polymerization in the presence of a catalyst. The catalysts used herein are basic substances, for example, potassium hydroxide, cesium hydroxide, quaternary ammonium hydroxides, and quaternary phosphonium hydroxides. At the end of polymerization of organopolysiloxanes, there is obtained a crude product containing the catalyst. An acid donor such as ethylene chlorohydrin, trimethylchlorosilane, phosphoric acid, carbonic acid and propionic acid is added to the crude product to neutralize the basic substance as the catalyst for deactivation. A neutralization salt which may be an inorganic or organic salt is then formed as ionic crystals. Such ionic crystals are removed by washing or filtering the product. Organopolysiloxanes are purified in this way.
In the last stage, siloxanes are generally purified by admitting the siloxane product containing ionic crystals into a container with mechanical agitation means, adding an adsorbent (e.g., charcoal) or filtering aid (e.g., diatomaceous earth) to the product in an amount of several percents by weight, agitating the mixture at a certain temperature for a certain time, and separating and removing the adsorbent or filtering aid from the siloxanes. The same treatment may be continuously carried out using a fixed bed.
This purifying method, however, encounters an increased load in the filtration step intended for removal of the adsorbent or filtering aid. It is also accompanied by a substantial loss of siloxanes due to adhesion of siloxanes to the adsorbent or filtering aid and is less efficient in operation. The adsorbent or filtering aid used becomes an industrial waste which must be disposed of through a cumbersome treatment and is unwanted from environmental considerations. In practice, with this purifying method it is quite difficult to decrease the content of ionic crystals in siloxanes to a sufficiently low level. Therefore, this purifying method is very disadvantageous from the aspects of process, productivity, operation efficiency, environment, economy and quality.
Another method for purifying siloxanes known comprises adding more than about 10% by weight of water to siloxanes in a container, agitating the mixture at a certain temperature for a certain time, allowing the mixture to stand in a stationary state for incurring liquid-liquid separation into siloxanes and water, and thereafter separating the aqueous layer of liquid from the container. The procedure of these steps is done once or repeated two or more times until siloxanes are purified to a desired degree. It is also known that in order to promote the liquid-liquid separation into siloxanes and water, a suitable solvent such a isopropyl alcohol and toluene is added at this stage.
The purifying method mentioned just above, however, has the problem that it is very difficult in practice to achieve complete separation between the liquid of the aqueous layer and the siloxane. As a result, separation is incomplete and the siloxanes are recovered in lower yields. Also the method uses the vast amount of water, which increases a load for waste water disposal and is detrimental to the environment. The throughput of this method is so small that the method must be repeated in several batches, resulting in a lowering of productivity. It takes a long time to separate water from siloxanes, for example, at least one hour until a desired level of separation is achieved although the exact time depends on the properties of siloxanes to be purified and a scale of production. Further complete separation beyond this level is very difficult in practice. It is sometimes necessary to positively remove water using a dewatering agent such as sodium sulfate. Even when such positive removal of water is employed, it is still very difficult in practice to remove ionic crystals from siloxanes to a final trace level. Therefore, this purifying method is also very disadvantageous from the aspects of process, productivity, environment, economy and quality.
Where various organochlorosilanes alone or in admixture of two or more are converted through hydrolysis into cyclic, linear or branched organopolysiloxanes, the hydrolysis entails hydrochloric acid as a by-product. In order to remove the hydrochloric acid from the hydrolyzate at the end of hydrolysis, it is a common practice to subject the hydrolyzate to neutralizing water washing using a basic substance such as sodium carbonate. However, since it is very difficult to completely remove from the hydrolyzate the neutralization salt resulting from neutralizing water washing by using the aforementioned purifying method, and since it is impossible in fact to remove the neutralization salt to a final trace level, some neutralization salt often remains in the hydrolyzate. There is a problem that the presence of such residual neutralization salt can adversely affect the properties of the hydrolyzate. Removing the neutralization salt as much as possible requires increased cost and energy.
As discussed above, the prior art siloxane purifying methods suffer from the problem that ionic crystals are left in siloxanes to detract from the physical properties of siloxanes required in various applications including outer appearance, heat resistance, and electrical properties.